1. Field of the Invention
This invention relates to a magnetoinductive flow measuring method for measuring the flow of a medium traveling through a measuring tube, for which, with the aid of at least one field coil, a magnetic field is generated that contains a magnetic field component extending perpendicular to the axis of the measuring tube, the electric resistance of the field coil is measured and, as the first temperature, the temperature of the field coil is calculated on the basis of the measured resistance of the field coil. The invention further relates to a magnetoinductive flowmeter that serves to measure the volume flow of a medium traveling through a measuring tube, incorporating a field coil for generating a magnetic field with a magnetic field component that permeates the measuring tube in a direction perpendicular to the direction of flow, as well as an output device allowing the readout, as the first temperature, of the field-coil temperature calculated on the basis of the measured electrical resistance of the field coil.
2. The Prior Art
A magnetoinductive flow measuring method and a magnetoinductive flowmeter of the type referred to above have been described for instance in JP 08014971 A. According to that publication, the coil temperature in a magnetoinductive flowmeter is determined by measuring the voltage on the field coil. That is to detect changes in the flow rate which are caused by temperature changes of the medium flowing through the measuring tube.
In general, magnetoinductive flow measuring methods and magnetoinductive flowmeters have been well known for some time and have been employed in a variety of different fields of application. The underlying concept of a magnetoinductive flowmeter for measuring the volume flow of a medium traveling through a measuring tube goes all the way back to Faraday who in 1832 proposed applying the principle of electrodynamic induction for measuring flow rates. According to Faraday's Law of Induction, a medium that contains charge carriers and flows through a magnetic field will produce an electric field intensity perpendicular to the direction of flow and perpendicular to the magnetic field. A magnetoinductive flowmeter utilizes Faraday's Law of Induction in that a field coil generates a magnetic field that contains a magnetic field component perpendicular to the direction of flow. Within that magnetic field, each volume element of the medium flowing through the magnetic field and containing a certain number of charge carriers, will contribute the field intensity generated in the volume element concerned to the voltage that can be collected by way of measuring electrodes.
In conventional magnetoinductive flowmeters, the electrodes are designed either for conductive coupling with the flowing medium or for capacitive coupling with the flowing medium. A salient feature of magnetoinductive flowmeters is the proportionality between the measured voltage and the flow rate of the medium averaged across the diameter of the measuring tube, i.e. between the measured voltage and the volume flow.
There are some magnetoinductive flowmeters with a so-called idling detection feature, giving the user information on the fill level of the measuring tube. Then there are magnetoinductive flowmeters that give the user information on the composition of the medium flowing through the measuring tube along the definitions of “liquid” versus “gaseous”. This is particularly necessary for correct flow measurements involving strongly foaming substances.
There is also a magnetoinductive flowmeter, described in U.S. Pat. No. 4,651,286, that is equipped with an alarm unit that can emit an alarm signal when the field coil of the magnetoinductive flowmeter is either open or short-circuited. Finally, DE 101 18 001 C2 describes a magnetoinductive flowmeter and a method for operating that flowmeter, in which the temperature of the field coil, hereinafter the first temperature, is determined on the basis of the measured impedance of the field coil, and a shut-off device is provided by means of which the power supply to the field coil is disconnected when the first temperature of the field coil exceeds a predefined temperature level. This is to prevent temperature-induced damage to the magnetoinductive flowmeter, especially to its field coils, without requiring an additional temperature sensor.